Asphalt distributors apply hot liquid asphalt to road and other surfaces in a variety of paving applications. Upon cooling, asphalt material becomes more viscous and eventually "freezes" to provide a binder material for pavement. Although it is desired that asphalt freezes upon spray application, it is important to prevent cooling of the asphalt material before spraying, while the asphalt is in the distributor. If asphalt freezes in the distributor, the asphalt can cause serious operating problems such as plugging the system and decreasing uniformity of the spray application.
Asphalt distributors conventionally include a tank, a pump, a spray bar and the plumbing network for communicating asphalt from the tank through the pump to the spray bar. The tank, pump and spray bar are conventionally supported directly by a vehicle such as a truck or supported by a detachable trailer pulled behind the vehicle. The plumbing network of an asphalt distributor preferably performs a number of desired functions, including loading of asphalt into the tank, off-loading asphalt out of the tank, transfer to handspray operations, circulating asphalt in the tank during initial asphalt heating, spraying of asphalt, and circulating asphalt through the spray bar while not spraying to prevent freezing of asphalt therein.
A prior attempt of providing an asphalt distributor is exemplified by Hill, U.S. Pat. No. 4,274,586. Hill provides a circulating system that includes dual feed lines connecting the pump to the spray bar, each feed line being connected near one end of the spray bar. In Hill, flow through one feed line is positive or one directional while the flow through the other feed line is positive or negative (bidirectional) depending upon whether spraying or circulation through the bar is desired. During normal spraying operations, flow through both feed lines is positive to deliver asphalt flow to the spray bar. However, when the distributor is stopped, flow through the bidirectional feed line is typically reversed to circulate asphalt through the spray bar and back to the tank to continuously move the asphalt through the feed lines and spray bar and prevent asphalt from freezing therein. Switching the flow is accomplished with an intermediate conduit having an on/off valve therein selectively connecting the feed lines, an adjustable pressure relief valve in the bidirectional feed line, and a pair of on/off valves in the spray bar.
Problems existing in the art relate to the complexity and cost of providing the circulating network in the asphalt distributor. Prior attempts have typically required complex and multiple valves and extensive lengths of circulating plumbing to reverse the flow of asphalt in one of the feed lines and provide the desired operating functions of an asphalt distributor, while all the time preventing asphalt from freezing and plugging the system. Not only are complex valves expensive but the multiple valve locations which are dictated by the routing of interconnecting plumbing do not provide easy operation or straightforward understanding of operation. For manually operated valves, this requires extra worker training and presents a potential safety hazard. The multiple connections can be prone to assembly difficulties and leaks, and the multiple lengths of exposed plumbing result in excessive heat loss from the asphalt which can lead to freezing or plugging of the system.